563 ARTICLE Long-term impact of a leaf miner outbreak on the performance of quaking aspen Diane Wagner and Patricia Doak Abstract: The aspen leaf miner, Phyllocnistis populiella Cham., has caused widespread and severe damage to aspen in the boreal forests of western North America for over a decade. We suppressed P. populiella on individual small aspen ramets using insecticide at two sites near Fairbanks, Alaska, annually for 7 years and compared plant performance with controls. Insecticide treatment successfully reduced leaf damage by P. populiella during most years and had little effect on herbivory by externally feeding invertebrates. By the end of the study, control ramets had suffered a reduction in height and girth relative to treated ramets and to the original, pretreatment size. Control ramets produced smaller leaves during some years and, after 7 years, produced fewer total shoots and leaves than ramets sprayed with insecticide. Treatment did not affect mortality, but at the warmer of the two sites, ramets sustaining ambient levels of leaf mining were significantly more likely to die back to basal sprouts than those treated with insecticide. We conclude that a decade of P. populiella outbreak has caused strongly negative effects on aspen development and the production of aboveground tissues. Résumé : La mineuse serpentine du tremble (Phyllocnictis populiella Cham.) cause des dommages sévères et très répandus dans les forêts boréales de peuplier faux-tremble de l'ouest de l'Amérique du Nord depuis plus de 10 ans. À chaque année pendant 7 ans, nous avons éliminé la mineuse sur de petits ramets de peuplier faux-tremble a` l'aide d'un insecticide dans deux stations près de Fairbanks, en Alaska, et nous avons comparé la performance des plants traités a` celle de plants témoins. L'insecticide a réussi a` réduire les dommages foliaires causés par la mineuse durant la plupart des années et a eu peu d'effet sur l'herbivorisme par les invertébrés qui se nourrissent des feuilles. À la fin de l'étude, les ramets témoins avaient subi une réduction en hauteur et en circonférence relativement aux ramets traités et a` leur dimension originale avant le traitement. Les ramets témoins ont produit de plus petites feuilles durant certaines années. Après sept ans, ils avaient produit au total moins de pousses et moins de feuilles que les ramets traités avec un insecticide. Le traitement n'a pas eu d'effet sur la mortalité mais, dans la station la plus chaude, les ramets qui subissaient les attaques régulières de la mineuse étaient significativement plus susceptibles de dépérir jusqu'au rejet basal que ceux qui avaient été traités avec un insecticide. Nous concluons qu'une dizaine d'années d'épidémie de mineuse a eu des effets négatifs importants sur le développement du peuplier faux-tremble et sur la production de biomasse aérienne. [Traduit par la Rédaction] For personal use only. Introduction often restricted to one or a few tissues within leaves (Hering 1951); Performance has declined and mortality increased for many consequently, injury caused by leaf mining may be less harmful to tree species across western North America during the past several the physiology of the plant than wholesale leaf removal. On the decades (van Mantgem et al. 2009). Recent regional growth de- other hand, defoliation may provide opportunities for growth clines and dieback of quaking aspen (Populus tremuloides Michx.) compensation that leaf mining damage does not. Studies of aspen are a particular source of concern among biologists and land man- and the tropical palm Chamaedorea elegans Mart., for example, in- agers (Huang and Anderegg 2012). Although much attention has dicate that defoliated plants can benefit from a rapid increase in focused on arid regions of the southwestern US, declines in light penetration through the canopy, increasing photosynthesis growth and survival have been documented in the boreal forest as by the remaining leaves (Kruger et al. 1998; Anten and Ackerly well (Michaelian et al. 2011). Evidence from western Colorado in- 2008). In contrast, a rapid increase in light penetration is not dicates that dieback is the result of water stress, which causes typical of leaf-mining damage. While early abscission of mined hydraulic failure in roots and branches (Anderegg et al. 2012). leaves is common across several broadleaf tree species, mined However, dendroecological studies of aspen in the Canadian bo- leaves typically remain on the plant during the height of the real forest indicate that attack by phytophagous insects may also growing season and abscise late in the summer (e.g., Pritchard and play a role in dieback (Frey et al. 2004). James 1984; Stiling and Simberloff 1989; Salleo et al. 2003; Wagner Outbreaks of insect herbivores can have significant negative et al. 2008), providing relatively little opportunity for photosyn- effects on plant performance (Yang 2012). However, some plant thetic compensation due to an increase in light availability. populations have considerable tolerance for herbivory (Strauss As of 2012, aspen in the boreal forests of Alaska and the Yukon and Agrawal 1999), even during insect outbreaks (Kessler et al. Territories had sustained high levels of foliar damage by the epi- 2012). The vast majority of our current understanding of the effect dermal leaf miner, Phyllocnistis populiella Cham., for more than a Can. J. For. Res. Downloaded from www.nrcresearchpress.com by Biosciences Library (Univ Alaska Fairbanks) on 08/25/14 of insect outbreaks on plant performance comes from the study of decade (USDA Forest Service 2012; Yukon Energy, Mines and defoliating insects. Population outbreaks of leaf-mining insects, Resources, Forest Management Branch 2012). At the peak of the out- while less common and less well-studied than defoliator out- break in 2007, P. populiella infested over 300 000 ha of Alaskan breaks, can also have negative effects on growth and reproduction forest. During the same year, 100% of the aspen trees surveyed of the host (e.g., Raimondo et al. 2003). Feeding by leaf miners is near Fairbanks, Alaska, sustained leaf-mining damage, which Received 29 November 2012. Accepted 18 April 2013. D. Wagner and P. Doak. Institute of Arctic Biology and Department of Biology and Wildlife, University of Alaska Fairbanks, Fairbanks, AK 99775-7000, USA. Corresponding author: Diane Wagner (e-mail: [email protected]). Can. J. For. Res. 43: 563–569 (2013) dx.doi.org/10.1139/cjfr-2012-0486 Published at www.nrcresearchpress.com/cjfr on 22 April 2013. 564 Can. J. For. Res. Vol. 43, 2013 Table 1. Average (± SE) climate characteristics recorded near the two study sites in interior Alaska, Bonanza Creek Long Term Ecological Research Area (BNZ) and Ester Dome (ED), between 2005 and 2011. Max snow Temperature (°C) Precipitation (mm) depth (mm) Month BNZ ED BNZ ED BNZ ED Apr. 1.8±0.8 0.0±0.9 6.1±4.0 8.4±1.6 May 11.3±0.5 9.2±0.6 15.1±4.9 19.2±5.5 June 14.7±0.3 13.3±0.3 53.9±6.1 54.8±8.2 July 16.2±0.6 14.6±0.7 63.3±11.8 69.9±13.2 Aug. 12.9±0.6 12.0±0.9 59.7±10.8 70.0±13.9 Growing season average 11.5±0.5a 9.8±0.5b 40.9±2.9a 42.1±2.2a Winter average 505±50a 718±89b Note: Measurements were made at Bonanza Creek Long Term Research Station LTER1, 3.8 km from the BNZ study site (elevation 352 m) and climate station USC00502868 near the apex of Ester Dome, 0.9 km from the ED study site (elevation 664 m). Data were obtained from the Bonanza Creek Long Term Ecological Research Program and the Alaska Climate Research Center. Growing season average temperature, average precipita- tion, and maximum snow depth were compared between sites with paired t tests; different letters indicate significant differences (n = 7 years, P < 0.05). extended over an average 58% of the total leaf surface area (SD 31, Aspen reproduce asexually, as well as sexually, and can form n = 743 leaves on 124 trees across 4 sites; unpublished data). Pre- clonal stands. We did not analyze the genetic identity of the focal vious work demonstrated that leaf mining by P. populiella impairs ramets in this study. There were no obvious breaks in aspen den- photosynthesis and causes early leaf abscission, thereby slowing sity or morphology within sites that would suggest clonal bound- aspen growth rates as measured over 3 years (Wagner et al. 2008). aries, and recent genetic work indicates that identification of The goal of the present study was to assess the effects of the aspen clones based on morphology or proximity to other trees is P. populiella outbreak on aspen mortality, dieback, and aboveg- inaccurate (Wyman et al. 2003). Aspen stands usually contain round tissue production over a longer period of time. more than one genotype, and the median size of monoclonal stands is estimated to be much smaller than the area of our study Materials and methods sites (Namroud et al. 2006; Mock et al. 2008; De Woody et al. 2009). Natural history of the herbivore Experimental design Phyllocnistis populiella is a univoltine, gracilliarid moth native to To test the effects of the leaf miner P. populiella on long-term North America. A specialist on the genus Populus, it has been aspen performance, we sprayed aspen ramets with insecticide collected throughout much of the US and Canada (Condrashoff annually for 7 years and compared survivorship, prevalence of 1964). In Alaska, both larvae and adults feed on Populus tremuloides, dieback, and aboveground tissue production with controls. In the larvae consuming leaves and adults consuming extrafloral May of 2005, we chose 40 aspen ramets 0.5–2 m tall at each site nectar.